Saufnase's Tutorial on Face Swapping in Photoshop
All the pictures POP to full size.

Alright, lets get started.
Roughly select the face in your source picture

Cut and paste it over into your destination picture as a new layer.
Right click on the new layer and select "Blending options". Change the Opacity to 50% so you can see the underneath picture.
Select Edit -> Free Transform (Ctrl+T) to resize and rotate the layer so the eyes align perfectly with the eyes in the underneath picture. Make sure the lower lip also aligns somewhat well with the underneath layer.
Now, change the Opacity back to 100% and select the erase tool (E). For this Picture I choose a very soft 40px diameter brush to roughly remover everything around the face.
Create a new adjustment layer "Hue/Saturation" and remove the saturation completly. This is not all that important, but for me it's much easier to judge brightness and contrast in black and white then in color. And that's why I do this step.
Now go back and select the layer with the new face and choose from the menu Image -> Adjustments -> Levels (Ctrl+L). Play around with all the sliders to make it look as seamless as possible. That might be a bit tricky in the beginning but as soon as you get a feeling on how all the sliders modify the picture you get results faster.
Remove or deselect the Hue/Saturation adjustment layer and select Image -> Adjustments - Hue/Saturation from the menu. Since the Brightness and contrast should be fine now, just adjust the "Hue" and "Saturation" sliders to match the rest of the face.
Do some more erasing and/or smudging in areas witch don't look seamless ...

Add necessary accessories ...

Done. :o)